Step 1: Materials
Blue roll of semi clear wrapping
Styrofoam sheet from HDepot
Grey sweat shirt and pants
Hot glue - lots of it
Grey spray paint
Hot glue gun
Misc. shop tools
Step 2: The Build
Begin by getting your hands on the Lego rock monster toy. I'm guessing if your kid wants to be this you already have a few. Then cut out and form a cardboard tub to fit your child. Determine the amount of space needed for the head opening and then cut a Styrofoam ring for the top to hold the cylinder shape.
Draw on and cut out the hands from the foam. Stack 3 sheets high and cut out area for pringles tube. Hot glue pieces together and repeat for other hand.
Cut to size and hot glue a cardboard insert pieced for the hands. This adds strength and helps look more like the toys hand. Roughly shape the hands to look like the toy.
Cut out a few circles and glue together to start building up the head piece.
Take measurements of your child's head and build up as needed.
I shaped out the inside of head piece and lined it with cardboard strips (keeps Styrofoam out of the hair and looks cleaner)
Rough cut rock looking shapes using a Dremel or other tool. This is a good place to use the toy for a reference.
I recommend reinforcing the inside mouth opening with cardboard strips and then cut out teeth shapes.
Stack and glue up 2,3 and 4 high curved pieces of different shapes for rocks. Sand at different angles like the toy. Where a mask and glasses because this foam can't be good for you to breath and it gets everywhere.
Step 3: More Building
Now cut out arm holes, trimming and securing the wrapping paper as necessary.
Sand all of your Styrofoam pieces smooth and number or mark where they fit the best. Leave extra space around the arms for movement.
After you have all of your rocks cut, sanded and know where they are going to fit. Paint everything grey or whatever color you choose. Test a sample to make sure the paint doesn't eat away your Styrofoam.
Hot glue on as many rocks as you want. If you can figure out a better way to secure them go for it. My son lost two rocks on Halloween night but it wasn't a problem.
Instead of glueing the head piece to the body I put a few pieces of Velcro along the edges in case it needs to come off.
Now you can cut off the sleeves from the sweatshirt and hot glue/tape them into arm holes. Give yourself plenty of extra material to glue. You can trim it off later.
I decided to attach the hands to the sleeves to keep the costume together but you don't have to. I was going to put foam in the arms to make it look rocky but it would have been difficult to get in and out of the costume.
Build up a few pieces of Styrofoam to match the shape of the rock monsters legs. Sand as needed and wrap in blue material securing with hot glue. Attach these to the sweat pants with black tape so they move freely with your child's legs.